The goal of this proposal is to define the in vivo role of trophinin in embryo implantation using the mouse as a model. The specific aims are: 1) to define the in vivo role of trophinin by analyzing trophinin gene knockout mice; 2) to define the expression and detailed localization of trophinin in mouse blastocysts and endometrium by determining the localization of trophinin in mouse blastocyst and endometrial epithelium at peri-implantation periods by immunoelectron microscopy, 3) to define the interaction between trophinin and tastin by yeast two-hybrid assays, and 4) to define the promoter activity of the human trophinin gene in order to understand the temporal and spatial expression of the trophinin in the cells involved in implantation.